Simple exchange mechanism

Although the accelerator compounds can form copper complexes at sufficiently high pH values,167 171 it is unlikely that the catalytic action operates by a simple exchange mechanism such as... 

The simple hopping mechanism, however, is not the only possible way for atoms to move on Cu(100). Figure 6.11 illustrates another possible diffusion process known as an exchange mechanism. In this mechanism, the adatom (shown in dark grey), replaces an atom that is initially a surface atom (shown in light grey) while the surface atom pops up onto the surface,... 

Simple Exchange Reactions. If an examination of the features of the exchange of a particular hydrocarbon indicates that a simple exchange process is operating, the following possible mechanisms must be considered. 

Multiple Exchange Reactions. The possible mechanisms which may lead to multiple exchange are necessarily more complicated than those responsible for simple exchange. Four alternatives may be considered. 

The kinetics and dynamics of crvptate formation (75-80) have been studied by various relaxation techniques (70-75) (for example, using temperature-jump and ultrasonic methods) and stopped-flow spectrophotometry (82), as well as by variable-temperature multinuclear NMR methods (59, 61, 62). The dynamics of cryptate formation are best interpreted in terms of a simple complexation-decomplexation exchange mechanism, and some representative data have been listed in Table III (16). The high stability of cryptate complexes (see Section III,D) may be directly related to their slow rates of decomplexation. Indeed the stability sequence of cryptates follows the trend in rates of decomplexation, and the enhanced stability of the dipositive cryptates may be related to their slowness of decomplexation when compared to the alkali metal complexes (80). The rate of decomplexation of Li" from [2.2.1] in pyridine was found to be 104 times faster than from [2.1.1], because of the looser fit of Li in [2.2.1] and the greater flexibility of this cryptand (81). At low pH, cation dissociation apparently... 

Figure 3.28 reduces to the simple mechanism of figure 3.27 if both Kx and K Y are very small. If Kx is small (i.e. the solute molecule is mainly in the mobile phase) but K Y is large (the pairing ion is mainly absorbed into the stationary phase), then the mechanism of retention in IPC becomes similar to that of IEC. Typical ion-pairing as well as typical ion-exchange mechanisms may play a role in practical IPC systems. 

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